# Molecular Lineage Replacement and Shifted Seasonality of Pediatric Respiratory Syncytial Virus on Tropical Hainan Island, China, 2021–2024

## TL;DR

This study tracks how RSV resurged in China's tropical Hainan Island after pandemic restrictions eased, showing changes in seasonality and virus strains.

## Contribution

The study reveals molecular lineage replacement and shifted seasonality of RSV in a tropical region post-pandemic NPI relaxation.

## Key findings

- RSV positivity rebounded sharply in 2023–2024 after being suppressed in 2022 due to NPIs.
- Seasonality shifted from summer–autumn to spring–summer, with RSV-A dominating in 2021–2023 and RSV-B in 2024.
- Lineage-specific amino-acid and glycosylation changes were observed in RSV-A and RSV-B strains.

## Abstract

Respiratory syncytial virus (RSV) resurged in many regions after the relaxation of stringent non-pharmaceutical interventions (NPIs) implemented during the COVID-19 pandemic. Here, we characterized the epidemiological patterns and molecular evolution of RSV among pediatric inpatients with acute respiratory tract infections (ARTIs) on tropical Hainan Island, China. We retrospectively analyzed 32,329 children (≤18 years) hospitalized at Hainan Women and Children’s Medical Center from January 2021 to December 2024. RSV positivity was determined using targeted next-generation sequencing. In total, 4483/32,329 (13.86%) patients were RSV-positive, with a high positivity in 2021 (20.27%, 957/4721), marked suppression in 2022 (2.03%, 106/5227) during intensive NPIs, and a rebound in 2023–2024 (15.31%, 1490/9732; 15.26%, 1930/12,649). RSV positivity was higher in boys than girls (14.42% vs. 13.00%). Seasonality shifted from a summer–autumn peak in 2021 to a spring–summer predominance in 2023–2024. Among 56 sequenced RSV-positive specimens (29 RSV-A; 27 RSV-B), all RSV-A strains belonged to genotype ON1 (lineages A.D.3 and A.D.5.2), and all RSV-B strains belonged to genotype BA9 (lineages B.D.4.1.1, B.D.E.1, and B.D.E.2). Subtype dominance transitioned from RSV-A (2021–2023; mainly A.D.3) to RSV-B in 2024 (all B.D.E.1). Lineage-specific amino-acid and predicted N-glycosylation changes were observed, including loss of the N179 site in A.D.5.2 and acquisition of N258 in B.D.E.1. These findings indicate that RSV circulation on tropical Hainan was strongly suppressed during intensive NPIs and re-established after policy relaxation, accompanied by earlier seasonal activity and clear lineage replacement, underscoring the need for sustained genomic surveillance to inform locally tailored clinical preparedness and immunization strategies.